1. Field of the Invention
This invention relates to a polyolefin film structure with improved barrier properties. In one aspect, the invention concerns a method of making a multilayer biaxially-oriented polypropylene film by incorporating in a core layer a minor amount of wax to improve the moisture barrier and haze properties thereof.
2. Description of Related Art
Biaxially-oriented polypropylene (BOPP) films are widely used in packaging because they have good stiffness, strength, optical properties (low haze and high gloss), and moisture barrier properties. Good resistance to moisture transmission, as measured by the "moisture vapor transmission rate" ("MVTR"), is quite desirable in packaging of food items containing salt or sugar or ingredients that are inherently hygroscopic. Conversely, an item packaged with a specific, intended moisture content will be better able to maintain that required moisture and not dry out if it is packaged with a high moisture barrier film. An essential purpose of the BOPP barrier film is to extend the package shelf life.
Since barrier properties increase with film thickness, a thicker film made either by direct film extrusion and orientation or by lamination of two or more films can be expected to have an improved (i.e., reduced) MVTR. Although these thicker films remain clear, haze and gloss properties can suffer. Small improvements come at significant expense, as film costs increase proportionately with thickness.
One important method to reduce MVTR in BOPP packaging films is to coat the film with a thin layer of metal, e.g., aluminum, which can result in four- to ten-fold improvements in MVTR values. Such metallized film then becomes opaque, however, and the package contents cannot be viewed.
Resistance to moisture and especially oxygen transmission can also be improved by coating a BOPP film with poly(vinylidine chloride) ("PVDC"), or less commonly by coextruding a layer of PVDC or its copolymers with polypropylene, followed by orientation of the film. While such films are useful, they have several disadvantages. First, an adhesive or tie-layer is often needed to prevent delamination of layers of these two incompatible materials. Second, relatively thin coatings of PVDC provide an MVTR reduction of only a factor of two or three at best. Finally, the chloride in PVDC discourages recycling of the films.
Packaging films incorporating layers of poly(vinyl alcohol) ("PVA") or copolymers of ethylene and vinyl alcohol ("EVOH") can provide excellent oxygen barrier properties, but are unsuitable for high moisture barrier. The PVA or EVOH layers must be sandwiched between polyolefin layers to protect the PVA or EVOH from absorbing moisture and thereby losing the oxygen barrier. As with PVDC films, these films also require adhesive layers or tie-layers to prevent delamination.
Yet another approach, as taught in U.S. Pat. No. 4,921,749 to Bossaert et al., assignors to Exxon Chemical Patents Inc., has been to add to a polypropylene base layer a low molecular weight modifier such as hydrogenated petroleum resin in an amount from 3 to 30% by weight. The film is then biaxially-oriented and is said to achieve a 40% reduction in MVTR. However, this technique is subject to evolution of smoke and plateout on equipment during production, and is relatively costly at the concentrations of hydrocarbon resin used.
More recent work in U.S. Pat. No. 5,155,160 to Yeh et al. shows that the barrier properties of polyolefin films can be reduced by a factor of four or more if a partially incompatible wax is added, generally in an amount between about two and twelve weight percent, to the polyolefin.
When the materials taught by Bossaert et al. '749 and by Yeh et al. '160 are added to polypropylene film, particularly at the higher percentages taught therein, film shrinkage becomes an issue. In most packaging applications, good film dimensional stability is desired to avoid distortion of the package. These additives often migrate to the film surface where they may transfer to and "plate out" on packaging equipment or the goods themselves. Moreover, the surface properties of the film such as the coefficient of friction (slipperiness), film stiffness, heat seal characteristics, and printability can be adversely affected.